THE CONSTITUTION OF NATURE. 15 



otherwise arranged. Here we have atoms between which 

 a strong attraction is exercised, and also atoms between 

 which a weak attraction is exercised. One atom can jostle 

 another out of its place in virtue of a superior force of at- 

 traction. But though the amount of force exerted varies 

 thus from atom to atom, it is still an attraction of the same 

 mechanical quality, if I may use the term, as that of grav- 

 ity itself. Its intensity might be measured in the same 

 way, namely, by the amount of motion which it can impart 

 in a certain time. Thus the attraction of gravity at the 

 earth's surface is expressed by the number thirty-two, be- 

 cause, when acting freely on a body for a second of time, 

 it imparts to the body a velocity of thirty-two feet a second. 

 In like manner the mutual attraction of oxygen and hydro- 

 gen might be measured by the velocity imparted to the 

 atoms in their rushing together. Of course such a unit of 

 time as a second is not here to be thought of, the whole 

 interval required by the atoms to cross the minute spaces 

 which separate them not amounting probably to more than 

 an inconceivably small fraction of a second. 



It has been stated that when a body falls to the earth 

 it is warmed by the shock. Here we have what we may 

 call a mechanical combination of the earth and the body. 

 Suffer the falling body and the earth to dwindle in imagi- 

 nation to the size of atoms, and for the attraction of grav- 

 ity substitute that of chemical affinity, which is the name 

 given to the molecular attraction, we have then what is 

 called a chemical combination. The effect of the union in 

 this case also is the development of heat, and from the 

 amount of heat generated we can infer the intensity of the 

 atomic pull. Measured by ordinary mechanical standards, 

 this is enormous. Mix eight pounds of oxygen with one 

 of hydrogen, and pass a spark through the mixture ; the 

 gases instantly combine, their atoms rushing over the little 

 distances between them. Take a weight of forty-seven 



